To increase the noise proof performance of a cover with aperture excited by diffuse acoustic fields from the outside, it is a problem to prevent the occurrence of the inner acoustic mode at the lower frequencies. Authors had suggested the feed forward active noise control (ANC) for the acoustic field inside the cover. Diffuse acoustic field excitation experiment using reverberant room has been examined to comprehend acoustic characteristics inside the cover. The results show that the inner cover field is in the state of locally coherent at the frequencies of inner acoustic mode, so that it is good environment for the active noise control. Then, a calculation with two-dimensional finite difference time domain (FDTD) method has been done to examine the feasibility of feed forward ANC. The results show that it is possible to realize ANC for the acoustic field inside the cover by proper design of control filter. Finally, experimental measurement of diffuse acoustic field excitation in the reverberant room has been done again to examine the ANC with the design method mentioned above. The results show that feed forward ANC can reduce noise level by 5dB at the peak frequency caused by the inner acoustic mode. Furthermore, the inner acoustic pressure distribution is also reduced uniformly at the frequency. Therefore, it is concluded that it is possible to realize feed forward active noise control for the acoustic field inside the cover with aperture excited by diffuse acoustic fields from the outside, even if the excitation is random or the causality of I/O is not satisfied.